Originally Posted By: Shannow
Originally Posted By: AcuraTL
Awesome stuff.
About the big end deforming especially on the exhaust stroke, very true, which is why when an engine throws a rod from excessive rpm it's usually on the exhaust stroke and not the power stroke as many assume. Its also why turbocharging can actually help in that department slightly in some cases.
Great points...which leads into my latest distraction, trying to find as many papers on what happens to big ends on cylinder de-activation.
Here's a paper
https://dspace.lboro.ac.uk/dspace-jspui/...K_JMBD_2014.pdf
Clearly the captured gasses load the bearing.
Also, the active cylinder(s) work harder (check out the bearing temperature rising to 165C in spots)...conclusion was that bearing friction on cylinder deactivation was moot, the benefits are due to the BSFC of burning fuel more efficiently.
Now, put that all together, and some not small part of the temperature GENERATED in the big end is conducted through the crankshaft to be dissipated at the mains...that's where I've landed this week, and will explain more later as I can produce a cogent picture.
I know this is old but this is my first time back since it was posted. Just wanted to say thanks, that was a very interesting read. Some of it was predictable but some was a surprise for me.
I knew there would be some "compression" going on during any upstroke but I'm surprised to see just how much and coupled with twice as many cylinder pressure spikes along with virtually no less bearing loading you're 100% right in where the fuel economy gains come from. I also wonder if any measurable efficiency is lost in CDA mode as the air is rarified on both downstrokes.
I can see where BSFC can improve in gas engines from CDA jut by having to run the active cylinders harder, resulting in less manifold vacuum and higher dynamic compression and possibly better thermal efficiency. I wonder if ring drag reduction is enough in the deactivated cylinders to matter. I know they're still sliding but at least the top ring isn't being pushed against the cylinder as hard. Is it one big reduction in pumping losses or small cumulative improvements in pumping losses, ring drag, thermal efficiency, etc.
One thing is for sure, I'm at a loss in seeing how the Diesel engine in the paper saw any significant improvements in efficiency. Would it come in the form of slightly better thermal efficiency in the harder working active cylinders? If turbocharged Its a little easier, I guess, to see improvements in the form of more boost which would give the same benefits of cracking the throttle open farther in the gas engine. Are the Diesel engine BSFC gains mostly limited to turbocharged versions?
I would love to see the some thing but for a gas engine. Specifically, the pressure rises on the up strokes of the dead cylinders. No one is going to lower CR these days because mpg is too important but I would like to see the difference between a NA high compression gas engine and low compression turbo gas engine. I'm wondering how the low mechanical compression ratio would effect rod bearing loading vs the higher compression version. I think the turbo engine would have room for the biggest gains in efficiency. With the deactivated cylinders the valves are shut and the air isn't getting compressed as much. The active cylinders will be working harder, with more intake pressure, giving a higher effective compression ratio. This assumes the lower mechanical CR actually has less losses on the "up" strokes. I say "up" because they're no longer the compression and exhaust strokes.
I'm done. I meant to write a short thank you for the link to the paper and I got carried away. I've bored myself to sleep twice while writing, I feel sorry for anyone who reads this stuff.
